A communication system is a facility that enables communication between two or more entities such as user terminal equipment and/or network entities and other nodes associated with a communication system. The communication may comprise, for example, communication of voice, electronic mail (email), text messages, data, multimedia and so on.
The communication may be provided by fixed line and/or wireless communication interfaces. A feature of wireless communication systems is that they provide mobility for the users thereof. An example of a communication system providing wireless communication is a public land mobile network (PLMN). An example of the fixed line system is a public switched telephone network (PSTN).
A cellular telecommunication system is a communications system that is based on the use of radio access entities and/or wireless service areas. The access entities are typically referred to as cells. Examples of cellular telecommunication standards includes standards such as GSM (Global System for Mobile communications), GPRS (General Packet Radio Servers), AMPS (American Mobile Phone System), DAMPS (Digital AMPS), WCDMA (Wideband Code Division Multiple Access), UMTS (Universal Mobile Telecommunication System) and cdma2000 (Code Division Multiple Access 2000).
A communication system typically operates in accordance with a given standard or specification which sets out what the various elements of a system are permitted to do and how that should be achieved. For example, the standard of specification may define if the user, or more precisely user equipment is provided with a circuit switched service of a packet switched service or both. Communication protocols and/or parameters which should be used for the connection are also typically defined. For example, the manner in which communication shall be implemented between the user equipment and the elements of the communication network is typically based in a predefined communication protocol. In other words, a specific set of “rules” on which the communication can be based needs to be defined to enable the user equipment to communicate via the communication system. Typically, given standards will also define how authentication and authorisation mechanisms as well as how accounting systems should work.
The term “service” used in this document should be understood to broadly cover any service which a user may desire, require or be provided with. The term will also be understood to cover the provision of complementary services. In particular, but not exclusively, the term “services” will be understood to include internet protocol multimedia IM services, conferencing, telephony, gaming, rich call, presence, e-commerce and messaging for example, instant messaging, SMS and email.
It is desirable to provide users with the ability to roam. Roaming allows a user associated with a home network to leave the home network and enter a network operated by for example a different operator. In particular, it would be desirable to allow a user to roam between GSM/GPRS/UMTS and cdma2000 networks. Such roaming should enable access to both the circuit switched network as well as the packet data network. GPRS/UMTS networks are based on the architecture and standards developed by the third generation partnership projects (3GPP). The architecture of cdma2000 networks has been developed by the 3GPP2 organization. The packet data network architectures of these networks differ. Currently, users that are homed in one network can not roam into networks of another type and access packet data services. One of the reasons is that the authentication and authorisation mechanisms as well as the accounting systems in GPRS UMTS and cdma2000 networks are quite different even if the mobile terminal had the necessary radio/air-interface technology.
Qualcomm has proposed one method to deal with this issue in “CDMA/GPRS roaming proposal”, by R. Hsu and J. Nasielski, Qualcomm, February 2004. In this proposal, an interworking and interoperability function (IIF) is used to convert all messages back and forth between the networks. The solution assumes that GGSNs in GPRS network supports FA (foreign agent) functionality and MIPv4. For performing Simple IP service in a GPRS network, the proposal establishes a PPP session via L2TP back to the cdma2000 home network. All data traffic is routed through the IIF and the home cdma2000 system. This proposal has certain problems. The solution does not utilize some of the more recent developments in the network architecture of 3GPP networks. Using a gateway such as the IIF to manage all messaging between the networks has a number of limitations:    1. The solution is complex. Emulating functions such as the GGSN, Foreign Agent, LAC and other functions adds a great deal of complexity. It also requires the gateway to maintain state for a large number of mobile users and hence scalability becomes an issue.    2. The solution is telecom centric. The architecture in the proposal is based on a roaming model that has existed in circuit-switched mobile networks for a brig time. Since the problem being addressed here is one of packet data network roaming, it is preferable to look at it from the perspective of how such roaming has been accomplished in ISP networks.    3. The solution has a single point of failure. An IIF becomes a single point of failure and hence not suitable for enabling such roaming. A more distributed architecture that utilizes the appropriate network elements is better suited.    4. The solution requires emulation. The IIF in this solution must emulate a number of different functions such as the Mobile IPv4 Foreign Agent, a GGSN and PDSN functions. Such emulation is tantamount to replicating the network elements that support packet data service in GPRS/UMTS and cdma2000 in the IIF.
A solution with minimum number of changes to the GPRS/UMTS and cdma2000 standards is needed in order to provide seamless roaming for subscribers roaming in between these networks.